Xfe seems to rely on the same capability of mounting/umounting and viewing that the underlying system has. When run as regular user, it will mount an ntfs partition when listed as ntfs but not as ntfs-3g. The command line mounting/umounting shows the same behavior. So it is not an issue with Xfe in and of itself, as it is simply not imposing more rights of the user on the system than the system environment allows.

This may or may not be what is causing your issue, but it is what I observed on my box.

how's the ntfs-3g.gz done? Just by unpacking ntfs-3g.tcz and running cpio, or are there any other differences?

A separate ntfs-3g.gz would take off >40% of the The Full/Lite Core images, and would bring it closer to the original Tiny Core Linux, in being the same files with just a different boot loader as well as offering some additional tczs already included.